Only add roles that the member does not have

[arqunis]

Should fix #1046.

This is applied in 0.9. The commit will be cherry-picked onto current and next when it is merged.

[SadiinsoSnowfall]

I think it would be better to add the roles only if the call to edit_member is successful (as it is done in the add_role method).

[SadiinsoSnowfall]

You could also create a Vec capable of holding all the current roles as well as the new ones.
The current code will cause a re-allocation of the roles vector nearly every time (if one of the new roles is not already contained in self.roles)